Transparent plastic compositions are useful for a number of applications including components for electronic and scientific equipment, point of purchase displays, packaging containers, etc., since they permit visual inspection of the items housed within the plastic. Such transparent plastic compositions are especially useful for the packaging of electronic components such as magnetic head assemblies and clean room applications. In addition, it is often desirable that the packages be reusable after cleaning. However, such transparent plastic materials typically have low conductivity and, as a result, tend to accumulate static electrical charges during their manufacture and use. Such static electrical charges may cause dust or other particles to adhere to the plastic material or even cause the plastic material to adhere to itself or other articles. Further, such static charges may also lead to functional damage in the performance of highly sensitive electronic components.
Accordingly, there is a need for substantially transparent moldable, thermoplastic multipolymer compositions which prevent the buildup of static electrical charges and can dissipate such charges in order to avoid the disadvantages of prior art compositions.
Electrostatic-dissipative multipolymer blends are known in the prior art. For example, U.S. Pat. No. 5,298,558 discloses a blend of polyvinyl chloride, a small amount of an impact modifier polymer such as ABS graft copolymer and an electrostatic dissipative amount of a chain-extended polyether. U.S. Pat. No. 4,775,716 discloses an antistatic thermoplastic composition of an ABS graft copolymer and an electrostatic dissipative composition comprising a copolymer of an epihalohydrin and an oxirane-containing comonomer.
EP 0 596 704 discloses electrostatic-dissipative polymer blends of a styrenic polymer such as ABS or MABS, an epihalohydrin copolymer and a polyalkylenelactone. WO 95/14058 discloses an antistatic thermoplastic composition comprising a MABS copolymer and an inherently antistatic thermoplastic urethane copolymer. Other prior art which mention the use of polyurethanes to confer electrostatic-dissipative properties on thermoplastic polymers include U.S. Pat. Nos. 5,159,053 and 4,179,479.
It is well known that when blending two polymers which are incompatible, the blend will contain large particles or "islands" of one polymer (the minor component) imbedded in the other polymer (the major component) with very little adhesion between the two polymers. This results in undesirable physical properties with the blend often exhibiting the worst properties of both components. It is well known that the use of a compatibilizer, i.e., a "bridge," allows the components to accept each other, thereby resulting in much smaller particles on the minor component, with good adhesion occurring between the two components, thus allowing stress transfer and hence better physical properties.
In the present invention, no compatibilizer is required because there is good inherent compatibility between the methyl methacrylate copolymer and the polyetheresteramide. This is readily seen from the data presented below in Table III in respect to the elongation @ break which is a measure of the toughness of the blend; the data below shows a comparison of the elongation @ break for a blend of a standard grade of a polymer of methyl methacrylate containing a small amount of methyl acrylate comonomer blended with a polyetheresteramide vs. a blend of the methyl methacrylate copolymer and the polyetheresteramide of the present invention.